Long term evolution (lte) system operating in an unlicensed spectral band with active network discovery and optimization of the unlicensed channels

ABSTRACT

A method for assigning a percentage of a CSAT time cycle to each radio node (RN) in a plurality of RNs that belong to a small cell radio access network (RAN) having a central controller includes: (i) for each time cycle period during which the RNs share a channel with one or more nodes that employ a different radio access technology (RAT), assigning a default occupancy percentage of the time cycles to each of the RNs; (ii) determining if the default occupancy percentage is able to be increased without violating one or more co-existence principles pre-established for the RAT employed by the RNs in the RAN and the different RAT; (iii) increasing the occupancy percentage of the first RN if it is determined that the default occupancy percentage is able to be increased without violating the co-existence principles; and (iv) sequentially repeating (ii)-(iii) for each remaining RN in the RAN.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application Ser.No. 62/297,199, filed Feb. 19, 2016 entitled ACTIVE NETWORK DISCOVERYAND OPTIMIZATION IN UNLICENSED CHANNELS, the contents of which areincorporated herein by reference in its entirety.

BACKGROUND

Operators of mobile systems, such as universal mobile telecommunicationssystems (UMTS) and its offspring including LTE (long term evolution) andLTE-advanced, continue to rely on advanced features to improve theperformance of their radio access networks (RANs). These RANs typicallyutilize multiple-access technologies capable of supportingcommunications with multiple users using radio frequency (RF) signalsand sharing available system resources such as bandwidth and transmitpower.

Recently, LTE systems have begun to extend their operation intounlicensed frequency bands such as the 5 GHz band, which is currentlyprimarily used by WiFi systems conforming to the IEEE 802.11specification. A technical specification being developed for the use ofLTE technology in unlicensed bands is referred to as LTE in Unlicensed(LTE-U). Because of the additional frequency resources that are madeavailable by the use of an unlicensed frequency band, it is possible toassign different, non-overlapping channels to different cells,simultaneously allowing system capacity improvements and reductions ininterference.

An important principle that is to be observed when operating LTE in anunlicensed band is to ensure that LTE-U co-exists with currenttechnologies such as Wi-Fi on a fair basis that allows both technologiesto use channels in that band. More particularly, one priority is thatLTE-U should not behave more aggressively toward an access point usingthe competing technology (e.g., Wi-Fi) than two access points using thecompeting technology would behave toward one another. That is, LTE-Ushould not degrade the performance of the competing technology any morethan would two interfering devices that both use the competingtechnology.

In general, coexistence mechanisms begin by selecting a channel in theunlicensed band that is currently not being used by the competingtechnology in order to avoid interference. A channel selection algorithmmonitors the operating channel on an on-going basis and will change to amore suitable channel if needed. If no unused channel is available, aCarrier Sensing Adaptive Transmission (CSAT) algorithm is used to applytime-division multiplexing based access (TDMA) techniques to LTE-Ucells, based on long-term carrier sensing of co-channel activities ofthe competing technologies. In this way the two technologies can sharethe channel fairly. In particular, CSAT defines a time cycle and theLTE-U cell transmits in a fraction of the cycle and gates off for theremainder of the cycle. The duty cycle of transmission vs. gating off isdictated by the sensed medium activity of the competing technology.

One issue that needs to be addressed when CSAT is employed as acoexistence mechanism concerns how the aforementioned co-existenceprinciples can be observed while optimizing system performance.

SUMMARY

In accordance with one aspect of the subject matter disclosed herein, amethod is shown for assigning a percentage of time to each radio node(RN) in a plurality of RNs that belong to a first small cell RAN havinga central controller operatively coupled to each of the RNs. Thepercentage of time is a percentage of time during which each of the RNsare able to occupy a channel shared by nodes employing a different radioaccess technology (RAT) from a RAT employed by the first RAN. Inaccordance with the method: (i) for each repetitive period defined by atime-division multiplexing access (TDMA) technique during which the RNsshare a channel with one or more nodes that employ the different RAT,assigning, with the central controller, a default occupancy percentageof the time periods to each of the RNs during which the RNs are able tooccupy the channel; (ii) determining, with the central controller, ifthe default occupancy percentage for a first of the RNs is able to beincreased without violating one or more co-existence principlespre-established for the RAT employed by the RNs in the first RAN and thedifferent RAT employed by the one or more nodes; (iii) assigning, withthe central controller, an increased occupancy percentage to the firstRN if it is determined that the default occupancy percentage for thefirst RN is able to be increased without violating the one or moreco-existence principles; and (iv) sequentially repeating (ii)-(iii) foreach remaining one of the RNs in the first RAN.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an enterprise in which a small cell radio access network(RAN) is implemented.

FIG. 2 is a flowchart illustrating a simplified example of a method foroperating an LTE-U system in an unlicensed frequency band.

FIG. 3 shows the duty cycle of an LTE-U cell operating in accordancewith a Carrier Sensing Adaptive Transmission (CSAT) algorithm.

FIGS. 4A and 4B each show network topologies in which two nodes are ableto interfere with one another.

FIGS. 5A and 5B each show network topologies in which three nodes arepresent in which the middle node is able to interfere with both of itsneighbors and, in FIG. 5A, two of the nodes belong to the same smallcell radio access network.

FIGS. 6A and 6B each show alternative network topologies in which threenodes are present in which the middle node is able to interfere withboth of its neighbors and, in FIG. 6A, two of the nodes belong todifferent small cell radio networks.

FIGS. 7A(I)-7C(III) show the results of three different decision rulesfor allocating channel occupancy percentages and transmission starttimes for the network topologies shown in FIGS. 6A and 6B.

FIG. 8 shows one example of a method that may be performed by a centralcontroller in a small cell LTE-U RAN to identify the existence of linksbetween a Wi-Fi AP and an external LTE-U.

FIG. 9 is a flowchart showing one example of a method that may beperformed by a central controller in a small cell LTE-U RAN to determinethe channel occupancy percentages that should be allocated to thevarious LTE-U RNs during CSAT time cycles.

FIG. 10 is a flowchart showing one example of a method that may beperformed by a central controller in a small cell LTE-U RAN to determinetransmission start times that should be assigned to the various LTE-Usduring CSAT time cycles once the percentages have been allocated.

DETAILED DESCRIPTION

Various systems, methods, and apparatuses are described in whichunlicensed spectrum is used for LTE communications. Various deploymentscenarios may be supported including a supplemental downlink mode ofoperation in which the LTE primary component carrier (PCC) uses thelicensed spectrum and the LTE-U secondary component carrier (SCC) usesthe unlicensed spectrum. More generally, in some implementationsdownlink and/or uplink traffic between a cell and a UE may be offloadedto an unlicensed spectrum. The unlicensed spectrum that is employed mayrange, by way of example and not as a limitation on the techniquesdescribed herein, from 600 Megahertz (MHz) to 6 Gigahertz (GHz).

Moreover, the techniques described herein are not limited to LTE-basedtechnologies (e.g., LTE-U, LAA), and may also be used for variouswireless communications systems such as CDMA, TDMA, FDMA, OFDMA,SC-FDMA, and other systems. The terms “system” and “network” are oftenused interchangeably. A CDMA system may implement a radio technologysuch as CDMA2000, Universal Terrestrial Radio Access (UTRA), etc.CDMA2000 covers IS-2000, IS-95, and IS-856 standards. IS-2000 Releases 0and A are commonly referred to as CDMA2000 1×, 1×, etc. IS-856 (TIA-856)is commonly referred to as CDMA2000 1×EV-DO, High Rate Packet Data(HRPD), etc. UTRA includes Wideband CDMA (WCDMA) and other variants ofCDMA. A TDMA system may implement a radio technology such as GlobalSystem for Mobile Communications (GSM). An OFDMA system may implement aradio technology such as Ultra Mobile Broadband (UMB), Evolved UTRA(E-UTRA), IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20,Flash-OFDM, etc. UTRA and E-UTRA are part of Universal MobileTelecommunication System (UMTS). LTE and LTE-Advanced (LTE-A) are newreleases of UMTS that use E-UTRA. UTRA, E-UTRA, UMTS, LTE, LTE-A, andGSM are described in documents from an organization named “3rdGeneration Partnership Project” (3GPP). CDMA2000 and UMB are describedin documents from an organization named “3rd Generation PartnershipProject 2” (3GPP2). The techniques described herein may be used for thesystems and radio technologies mentioned above as well as other systemsand radio technologies. The description below, however, describes an LTEsystem for purposes of illustration, and LTE terminology is used in muchof the description below, although the techniques are applicable beyondLTE applications. In this description, LTE-Advanced (LTE-A)communications are considered to be a subset of LTE communications, andtherefore, references to LTE communications encompass LTE-Acommunications.

FIG. 1 shows an enterprise 105 in which a small cell RAN 110 isimplemented. The small cell RAN 110 includes a plurality of radio nodes(RNs) 115 ₁ . . . 115 _(N). Each radio node 115 has a radio coveragearea (graphically depicted in the drawings as hexagonal in shape) thatis commonly termed a small cell. A small cell may also be referred to asa femtocell, or using terminology defined by 3GPP as a Home Evolved NodeB (HeNB). In the description that follows, the term “cell” typicallymeans the combination of a radio node and its radio coverage area unlessotherwise indicated. A representative cell is indicated by referencenumeral 120 in FIG. 1.

The size of the enterprise 105 and the number of cells deployed in thesmall cell RAN 110 may vary. In typical implementations, the enterprise105 can be from 50,000 to 500,000 square feet and encompass multiplefloors and the small cell RAN 110 may support hundreds to thousands ofusers using mobile communication platforms such as mobile phones,smartphones, tablet computing devices, and the like (referred to as“user equipment” (UE) and indicated by reference numerals 1251-N in FIG.1).

The small cell RAN 110 includes an access controller 130 that serves asa central controller managing and controlling the radio nodes 115. Oneexample of an access controller that operates in a mobile (small cell)RAN is the SpiderCloud services node, available from SpiderCloudWireless, Inc. The radio nodes 115 are coupled to the access controller130 over a direct or local area network (LAN) connection (not shown inFIG. 1) typically using secure IPsec tunnels. The access controller 130aggregates voice and data traffic from the radio nodes 115 and providesconnectivity over an IPsec tunnel to a security gateway SeGW 135 in anEvolved Packet Core (EPC) 140 network of a mobile operator. The EPC 140is typically configured to communicate with a public switched telephonenetwork (PSTN) 145 to carry circuit-switched traffic, as well as forcommunicating with an external packet-switched network such as theInternet 150.

The environment 100 also generally includes Evolved Node B (eNB) basestations, or “macrocells”, as representatively indicated by referencenumeral 155 in FIG. 1. The radio coverage area of the macrocell 155 istypically much larger than that of a small cell where the extent ofcoverage often depends on the base station configuration and surroundinggeography. Thus, a given UE 125 may achieve connectivity to the network140 through either a macrocell or small cell in the environment 100.

FIG. 2 is a flowchart illustrating a simplified example of a method foroperating an LTE-U system in an unlicensed frequency band. The methodmay be used by radio nodes, base stations and/or UEs such as the RNs 115and UEs 125 shown in FIG. 1, for example. In this example a RN uses themethod to provide downlink transmissions. First, in step 160 a channelis selected by a small cell based on LTE measurements. To make theselection, the small cell scans the unlicensed band in decision step 170and determines if a clean channel is available for the carriertransmission. This ensures that interference is avoided between thesmall cell and its neighboring Wi-Fi devices and LTE-U RNs in other RANscontrolled by different operators (referred to herein as external LTE-URNs), provided an unused channel is available. If a clear channel isfound, the small cell will transmit on that channel using the full dutycycle in step 180. The channel selection algorithm monitors the statusof the operating channel on an on-going base, and if needed will selecta more suitable one and change.

If no unused channel is available in step 170, a Carrier SensingAdaptive Transmission (CSAT) algorithm is used in step 190 to applytime-division multiplexing based access techniques to LTE-U cells, basedon long-term carrier sensing of co-channel activities of the competingtechnologies. In this way the two technologies can share the channelfairly. In particular, CSAT defines a time cycle and the LTE-U celltransmits in a fraction of the cycle and gates off for the remainder ofthe cycle. The duty cycle of transmission vs gating off is dictated bythe sensed medium activity of other devices and technologies. This useof a shared channel is depicted in the timeline shown in FIG. 3. Duringthe LTE-U OFF period 310, the channel is clean to neighboring Wi-Finodes, which can resume normal Wi-Fi transmissions. The small cell or UEwill measure the Wi-Fi medium utilization during the LTE-U off period310, and adaptively adjust the on/off duty cycle accordingly. The TDMcycle can be set to a few hundreds of msec, for instance, which caneffectively accommodate the activation/de-activation procedures whilecontrolling the data transmission delay.

As previously mentioned, one issue that needs to be addressed when CSATis employed as a coexistence mechanism concerns how the aforementionedco-existence principles can be observed while optimizing systemperformance. In particular, LTE-U systems operating on an unlicensedchannel should not degrade the performance of a competing technology(e.g., Wi-Fi) any more than would interfering devices that both use thecompeting technology. While mobile systems that operate in a distributedmanner may be able to ensure that these co-existence principles areobserved and accounted for locally, they cannot do so in a global mannerthat optimizes various performance metrics such as capacity. However,such optimization can be achieved in a mobile system that employs acentral processor such as the access controller 130 shown in FIG. 1 thatcan communicate with each RN 115 in the system.

Because the access controller 130 is in communication with the entireRAN, it is able to optimize the use of channels in unlicensed bandswhile treating the competing technology that uses those same bands on afair basis. Accordingly, the access controller may be used as part of aprocess for selecting the best CSAT strategy in one or more unlicensedchannels for use by different cells in a mobile system. Examples of anactive probing technique to discover the network topology and todetermine the globally optimal channel occupancy time (i.e., theduration of the ON period) and transmission timing are shown below. Ofcourse, this technique is not limited to the particular small cell RANor the particular access controller shown above, which are presented forillustrative purposes only. Moreover, the techniques described hereinare not limited to the particular problem of determining the CSATparameters discussed above, which are presented for illustrativepurposes only.

The advantages that can arise from the use of a central controller suchas access controller 130 will be illustrated with reference to FIGS. 4and 5. FIGS. 4A and 4B present a canonical example in which two nodes,Wi-Fi access point (AP) 210 and LTE-U RN 220, are sufficiently close sothat they are aware of one another and can interfere with one anotherwhen operating on the same channel. In order to determine how thechannel occupancy time of the CSAT time cycle is to be divided betweenthe two nodes so that the Wi-Fi AP 210 is treated as fairly by the LTE-URN 220 as it would be treated by another Wi-Fi AP, consider the caseshown in FIG. 4B, which replaces the LTE-U RN 220 with Wi-Fi AP 220′. Inthis case it is clear that the two nodes would share the channel equallyso that each is allowed to occupy the channel for half of the time.Accordingly, in FIG. 4A, the Wi-Fi AP 210 and LTE-U RN 220 should eachbe allowed to occupy the channel for half of the CSAT time cycle aswell.

FIGS. 5A and 5B show another, more complex situation in which there arethree nodes, Wi-Fi AP 235, LTE-U RN 245 and LTE-U RN 255. LTE-U RN 245is sufficiently proximate to both Wi-Fi AP 235 and LTE-U RN 255 so thatthey can interfere with one another and thus they cannot both transmitat the same time. However, Wi-Fi AP 235 and LTE-U RN 255 aresufficiently remote from one another that they are not aware of eachother and cannot interfere with one another. Once again, to determinehow the channel occupancy time is to be divided among the three nodes sothat the Wi-Fi AP 235 is treated as fairly by the LTE-U RNs 245 and 255as it would be treated by other Wi-Fi APs, consider the case shown inFIG. 5B, which replaces the LTE-U RNs 245 and 255 with Wi-Fi APs 245′and 255′.

In this case the impact is examined of four different decision rules onthe fraction of the channel occupancy time that is to be assigned toeach of the three nodes. Two of the rules are distributed decision ruleswhich illustrate how the channel occupancy time would be allocated ifeach node makes its own allocation decisions based on the neighboringnodes it is able to observe. A third rule is the decision that would bemade if all three nodes were Wi-Fi APs. The final rule illustrates thedecision that may be made a central controller (e.g., access controller130) that has access to the entire RAN in which the LTE-U RNs operate.The results of these rules for each node are presented below in Table 1.

TABLE 1 Decision Rule Left Node Middle Node Right Node D1 ⅔ ⅓ ½ D2 ⅓ ⅔ 1Wi-Fi Decision ⅔ ⅓ ⅔ Centralized Decision ⅔ ⅓ 1

Looking first at the Wi-Fi decision rule (row three in Table 1),consider first the middle Wi-Fi AP 245′ in FIG. 5B. Since this node seestwo other nodes that may interfere with it, it decides that it shouldoccupy ⅓ of the CSAT time cycle, which would therefore allow the leftWi-Fi AP 235′ to occupy ⅔rds of the time cycle. Looking next at theright Wi-Fi AP 255′, since the only node that interferes with it is themiddle Wi-Fi AP 245′ and that node occupies only ⅓rd of the time, theright Wi-Fi AP 255′ determines that it can occupy ⅔rds of the timecycle. Thus, this allocation of the CSAT time cycle as determined by theWi-Fi decision rule would be applicable to the three nodes shown in FIG.5A.

The first row in Table 1 illustrates the allocation of the time cyclefor the three nodes in FIG. 4A as determined in accordance with thefirst distributed decision rule D1. The premise of this rule is that allnodes that could interfere with one another should equally share theCSAT time cycle, regardless of the network operator of each node. Thefraction of the time cycle occupied by any node n, denoted f_(n), may bedetermined by the following formula:

$f_{n} = \frac{1}{{{all}\mspace{14mu} {neighboring}\mspace{14mu} {nodes}\mspace{14mu} {seen}\mspace{14mu} {by}\mspace{14mu} {node}\mspace{14mu} n} + 1}$

As shown in Table 1, this rule yields the same allocation for the Wi-FiAP 235 as the Wi-Fi decision rule and thus it is fair to the Wi-Fi AP235. Likewise, it also yields the same allocation as the Wi-Fi decisionrule for the middle LTE-U RN 245. However, the rule D1 only allocates ½of the channel occupancy time to the right LTE-U RN 255, which is lessthan that allocated by the next distributed decision rule D2.Accordingly, while decision rule D1 is fair to the Wi-Fi AP 235, it istoo conservative in that it does not maximize the spectral efficiencysince LTE-U RN 255 could occupy more of the CSAT time cycle than it hasbeen allocated.

The second row in Table 1 illustrates the allocation of the channeloccupancy time for the three nodes in FIG. 5A as determined inaccordance with the second distributed decision rule D2. The premise ofthis rule is that all nodes occupy a fraction of the channel occupancytime that is proportional to the number of LTE-U RNs that are managed bythe same network operator. The fraction of the time cycle occupied byany node n, denoted f_(n), in accordance with decision rule D2 may bedetermined by the following formula:

$f_{n} = \frac{{sum}\mspace{14mu} {total}\mspace{14mu} {of}\mspace{14mu} {all}\mspace{14mu} {intra}\text{-}{operator}\mspace{14mu} {LTE}\text{-}U\mspace{14mu} {nodes}\mspace{14mu} {seen}\mspace{14mu} {by}\mspace{14mu} {node}\mspace{14mu} n}{{{all}\mspace{14mu} {neighboring}\mspace{14mu} {nodes}\mspace{14mu} {seen}\mspace{14mu} {by}\mspace{14mu} {node}\mspace{14mu} n} + 1}$

As shown in Table 1, this rule only allocates ⅓^(rd) of the time cycleto the Wi-Fi AP 235 and thus it is not fair since it is being allocatedless time than the Wi-Fi decision rule would allocate. Accordingly, thisdecision rule is too aggressive with respect to the Wi-Fi AP 235 andthus is not acceptable.

The fourth and final row in Table 1 illustrates the allocation of thechannel occupancy time for the three nodes in FIG. 5A as determined by acentral controller such as access controller 130, which has completeknowledge of the RAN topology to which LTE-U RNs 245 and 255 belong.First, since the controller is able to determine that LTE-U RN 245 andWi-Fi AP 235 are able to interfere with one another but that Wi-Fi AP235 and LTE-U RN 255 are not able to interfere with one another, it willallocate ⅔rds of the channel occupancy time to the Wi-Fi AP 235 tosatisfy the co-existence principles. Since the central controller alsoknows that the right LTE-U RN 255 is not in the vicinity of the Wi-Fi AP235 and thus will not interfere with it, the central controllerallocates the entire time cycle to LTE-U RN 255.

Examining the four decision rules in Table 1 demonstrates that only thecentralized decision rule maximizes the efficient use of the unlicensedchannel while also not being more aggressive toward the Wi-Fi AP 235than another Wi-Fi AP would be.

FIGS. 4-5 illustrated the advantages of a centralized decision rule fordetermining the fraction of a CSAT time cycle during which the LTE-U RNsshould operate. Likewise, FIGS. 6-7 will be used to illustrate theadvantages of a centralized decision rule on the transmission starttimes for the LTE-U RNs during each CSAT time cycle. FIG. 6A shows asituation similar to that shown in FIG. 5A in which there are threenodes, Wi-Fi AP 310, LTE-U RN 320 and LTE-U RN 330. LTE-U RN 320 issufficiently proximate to both Wi-Fi AP 310 and LTE-U RN 330 so thatthey can interfere with one another. However, Wi-Fi AP 310 and LTE-U RN330 are sufficiently remote from one another so that they are not awareof each other and cannot interfere with one another. Unlike FIG. 5Ahowever, LTE-U RN 330 is an external node and thus LTE-U RN 320 andLTE-U RN 330 belong to different RANs controlled by different operators.

FIGS. 7A-7C show the results of the three different decision rules onboth the fraction of the time cycle allocated to each node and thetransmission start time for each node during a CSAT time cycle. In eachcase the top diagram (i) illustrates the behavior of the left Wi-Fi AP310 in FIG. 6A during a time cycle, the middle diagram (ii) illustratesthe behavior of the middle LTE-U RN 320 in FIG. 6A during a time cycleand the bottom diagram (iii) illustrates the behavior of the right LTE-URN 330 in FIG. 6A during a time cycle.

For purposes of illustration LTE-U RN 320 is assumed to be the node thatemploys the centralized decision rules. Accordingly, when ensuring thatWi-Fi AP 310 is treated by the LTE-U RNs as fairly as it would betreated by another Wi-Fi AP, an arrangement is considered in FIG. 6B inwhich the LTE-U RN 320 is replaced with the Wi-Fi AP 320′. The fractionof the CSAT time cycle allocated to the Wi-Fi AP 310 and Wi-Fi AP 320′(and hence to LTE-U RN 320) is shown in FIG. 7C. In this case, treatingthe middle LTE-U RN 320 as a Wi-Fi AP, the LTE-U RN 320 is allocated ⅓of the timing cycle since it can detect both of its neighboring nodes.As a consequence, Wi-Fi 310 is allocated ⅔ of the time cycle. The LTE-URN 330, using its own distributed decision rule, is only aware of LTE-URN 320 and hence is allocated ½ of the time cycle.

FIG. 7A shows the ON period durations and transmission start timesduring a time cycle for the three nodes of FIG. 6A in accordance with atypical distributed decision rule. Once again the allocations for theWi-Fi AP 310, LTE-U RN 320 and LTE-U RN 330 are ⅔, ⅓ and ½,respectively. However, the LTE-U RNs 320 and 330 start theirtransmissions at the same time during the time cycle, thus causinginterference. This may occur because although different LTE-U operatorsmay use different decision algorithms, they often use the same timingschedule and thus their transmission start times may be aligned.

FIG. 7B shows the ON period durations and transmission start timesduring a time cycle for the three nodes of FIG. 6A when the LTE-U RN 320employs a centralized decision rule that is implemented by a centralcontroller. Once again the fractional allocations for the Wi-Fi AP 310,LTE-U RN 320 and LTE-U RN 330 are ⅔, ⅓ and ½, respectively. However,because the central controller in communication with the LTE-U RN 320can detect the network topology of the external LTE-U RN 330 and theWi-Fi node 310, it is aware that LTE-U RNs 320 and 330 may interferewith one another. Accordingly, as shown in FIG. 7B, it can adjust thetransmission start time of the LTE-U RN 320 (without changing thefraction of the time cycle it occupies) so that it does not overlap withthe transmissions of either the Wi-Fi AP 310 or the LTE-U RN 330. Asalso shown in FIG. 7B, the transmission times of the Wi-Fi AP 310 andthe LTE-U RN 330 may overlap since they do not detect one another. Inthis way not only is the performance of the LTE-U RN 320 employing thecentralized decision algorithm improved, but the performance of theexternal LTE-U RN 330 is improved as well.

In the examples presented above it was important to know how Wi-Fi APswould allocate the channel occupancy time among themselves if the LTE-URNs were replaced with Wi-Fi APs. In order to determine this the Wi-FiAPs network topology first needs to be determined. This can be obtainedby the LTE-U network using network listening results. Once the topologyis available, along with the existence of external LTE-U that are notare part of the LTE-U network, any of a variety of algorithms may beemployed to determine the expected Wi-Fi AP channel occupancy times.

Network listening results may also be used to determine the networktopology of any external LTE-U RNs that may be present. However, oneproblem arises because, as previously mentioned, LTE-U network operatorsmay use the same timing schedule and thus their transmission start timesduring the CSAT time cycle may be the same. This is a problem becausethe network topology discovery process involves the RNs in one networklistening for signals from RNs in another network. If the RNs in bothnetworks are transmitting at the same time and then listening at thesame time during their synchronized OFF periods, they will not be ableto detect one another. This problem can be overcome with the use ofcentral controller. In particular, the central controller may iteratethrough each of the RNs in the RAN and sequentially cause them to be inthe OFF state when they would otherwise be transmitting. In this wayeach RN can listen for signals from any external LTE-U RNs that may bepresent.

The examples presented above assume that the LTE-U RAN knows whether theWi-Fi APs and the external LTE-Us that are not are part of the LTE-U RANare able to detect one another. For instance, in FIG. 6A, it was assumedthat no link exists between Wi-Fi AP 310 and LTE-U RN 330. Thisinformation can be inferred by the central controller in the LTE-U RANusing one LTE-U RN to transmit signals while the Wi-Fi APs in theproximity are operating and observing the changes in the behavior of theWi-Fi APs using either another LTE-U RN or the same LTE-U RN. Forinstance, consider again the topology in FIG. 6A and the results of thedistributed decision rule shown in FIG. 7A. As the results show, the ONperiod of LTE-U RN 320 does not overlap with the ON period of Wi-Fi AP310 thus preventing interference between them. To infer whether a linkexists between Wi-Fi AP 310 and LTE-U 330, the central controller in theLTE-U RAN adjusts the transmit start time of the LTE-U RN 320 so that itis delayed from the time shown in FIG. 7A (without changing the durationof its ON period). That is, the ON period for the LTE-U RN 320 isshifted or slid to the right so that it overlaps with the ON period ofthe Wi-Fi AP 310. The Wi-Fi AP 310 will then respond in one of two ways,depending on whether a link exists between it and the external LTE-U RN330. If no link exists, then the Wi-Fi AP 310 will respond by shiftingits own transmit start time, without changing its duration, so that itstransmission time does not overlap with the ON period of the LTE-U RN320. This is the behavior shown in FIG. 7B for Wi-Fi AP 310 and LTE-U RN320. A second LTE-U RN in the RAN (or RN 320 itself) can be used todetect this Wi-Fi AP behavior and report it to the central controller.On the other hand, if a link does exist between the Wi-Fi AP 310 and theexternal LTE-U RN 330 so that they may interfere with one another, thenthe total duration of the transmission time for the Wi-Fi AP 310 will beobserved by the second LTE-U RN in the RAN (or RN 320 itself) todecrease as a result of the interference. This procedure may be repeatedas necessary to determine whether links exist between any pairs of Wi-FiAPs and external LTE-U RNs.

The process described above that is employed by the LTE-U network toidentify the existence of links between a Wi-Fi AP and an external LTE-Umay be summarized by the flowchart shown in FIG. 8. First, at block 410,the internal and external network topology is determined using, forexample, radio environment monitoring (REM) scans that are performedunder the control of the central controller. Next, at block 420, the ONperiods for the internal LTE-U RNs are synchronized so that they startat e.g., the beginning of a CSAT time cycle. The central controller thenselects at block 430 LTE-U RNs that can detect the same at least oneexternal LTE-U RN and the same at least one Wi-Fi AP. For each pair ofLTE-U RNs that are selected, one of them is used to transmit whilevarying its ON period start time (but not its duration) at block 440,either continuously or in a limited number of steps, while the otherLTE-U RN is used to detect signals from a jointly detected Wi-Fi AP anddetermine whether the ON period of the Wi-Fi AP is decreasing. If atdecision block 450 the ON period decreases by more than some thresholdamount, then at block 460 a link is presumed to exist between the Wi-FiAP and the external LTE-U RN. Otherwise it is presumed at block 470 thatthe Wi-Fi AP and the external LTE-U RN are not aware of one another andthus will not interfere.

FIG. 9 is a flowchart showing one example of a method that may beperformed by a central controller (e.g., access controller 130) in asmall cell LTE-U RAN to determine the channel occupancy percentages thatshould be allocated to the various LTE-U RNs during CSAT time cycles.The method begins at block 510 when the channel occupancy percentagesare initialized to default percentages. In one embodiment thosepercentages may be determined in accordance with the Wi-Fi decisionrule. Next, at block 520 the central controller iterates through each ofthe LTE-U RNs in the RAN and examines whether the occupancy percentagescan be increased while satisfying the following two conditions: (1)ensuring that the occupancy percentages of any Wi-Fi APs as determinedby the Wi-Fi occupancy decision rule does not change; and (2) ensuringthat any external LTE-U RNs that are present are provided with a fairshare of the CSAT time cycle. In one embodiment, various distributeddecisions rules may also be executed and used as a reference todetermine which LTE-U RNs are candidates that may have their channeloccupancy percentages increased while satisfying the two conditionsspecified above.

In some embodiments the decision making process described in FIG. 9 mayemploy a closed-loop process in which various performance indictors maybe used to refine the percentage allocations and the transmission starttimes that are assigned to the LTE-U RNs. Examples of such performanceindicators may be obtained from various sources such as UE channelquality indicator (CQI) reports and radio resource control (RRC)measurement reports, for instance. These performance indicators may alsobe used to decide whether to fall back to the default occupancypercentages and transmission start times.

FIG. 10 is a flowchart showing one example of a method that may beperformed by a central controller in a small cell LTE-U RAN to determinetransmission start times that should assigned to the various LTE-U RNsduring CSAT time cycles once the percentages have been allocated. Themethod begins at block 610 when the transmission start times of the RANsare initialized to a common time, e.g., the start time of each timecycle. Next, at block 620, the transmission start times of the RNs areadjusted while ensuring that the medium utilization by any Wi-Fi APsdoes not significantly change and, if external LTE-U RNs are present,ensuring that they are provided with a fair share of the CSAT timecycle. The adjustments to the transmission start time may be continuousor quantized using a limited number of different timing positions. Next,at block 630, the central controller monitors the network throughput anddetermines if there has been any improvement. The monitoring may beaccomplished, for example, using the performance indicators such as CQIreports and the like, which were discussed above. Based on thismonitoring, the process can return to block 620 where additionaladjustments can be made to refine the transmission start times based onthe network throughput.

Several aspects of telecommunication systems will now be presented withreference to access controllers, base stations and UEs described in theforegoing description and illustrated in the accompanying drawing byvarious blocks, modules, components, circuits, steps, processes,algorithms, etc. (collectively referred to as “elements”). Theseelements may be implemented using electronic hardware, computersoftware, or any combination thereof. Whether such elements areimplemented as hardware or software depends upon the particularapplication and design constraints imposed on the overall system. By wayof example, an element, or any portion of an element, or any combinationof elements may be implemented with a “processing system” that includesone or more processors. Examples of processors include microprocessors,microcontrollers, digital signal processors (DSPs), field programmablegate arrays (FPGAs), programmable logic devices (PLDs), state machines,gated logic, discrete hardware circuits, and other suitable hardwareconfigured to perform the various functionalities described throughoutthis disclosure. One or more processors in the processing system mayexecute software. Software shall be construed broadly to meaninstructions, instruction sets, code, code segments, program code,programs, subprograms, software modules, applications, softwareapplications, software packages, routines, subroutines, objects,executables, threads of execution, procedures, functions, etc., whetherreferred to as software, firmware, middleware, microcode, hardwaredescription language, or otherwise. The software may reside on acomputer-readable media. Computer-readable media may include, by way ofexample, a magnetic storage device (e.g., hard disk, floppy disk,magnetic strip), an optical disk (e.g., compact disk (CD), digitalversatile disk (DVD)), a smart card, a flash memory device (e.g., card,stick, key drive), random access memory (RAM), read only memory (ROM),programmable ROM (PROM), erasable PROM (EPROM), electrically erasablePROM (EEPROM), a register, a removable disk, and any other suitablemedia for storing or transmitting software. The computer-readable mediamay be resident in the processing system, external to the processingsystem, or distributed across multiple entities including the processingsystem. Computer-readable media may be embodied in a computer-programproduct. By way of example, a computer-program product may include oneor more computer-readable media in packaging materials. Those skilled inthe art will recognize how best to implement the described functionalitypresented throughout this disclosure depending on the particularapplication and the overall design constraints imposed on the overallsystem.

1. A method for assigning a percentage of time to each radio node (RN) in a plurality of RNs that belong to a first small cell RAN having a central controller operatively coupled to each of the RNs, the percentage of time being a percentage of time during which each of the RNs are able to occupy a channel shared by nodes employing a different radio access technology (RAT) from a RAT employed by the first RAN, comprising: (i) for each repetitive period defined by a time-division multiplexing access (TDMA) technique during which the RNs share a channel with one or more nodes that employ the different RAT, assigning, with the central controller, a default occupancy percentage of the time periods to each of the RNs during which the RNs are able to occupy the channel; (ii) determining, with the central controller, if the default occupancy percentage for a first of the RNs is able to be increased without violating one or more co-existence principles pre-established for the RAT employed by the RNs in the first RAN and the different RAT employed by the one or more nodes; (iii) assigning, with the central controller, an increased occupancy percentage to the first RN if it is determined that the default occupancy percentage for the first RN is able to be increased without violating the one or more co-existence principles; and (iv) sequentially repeating (ii)-(iii) for each remaining one of the RNs in the first RAN.
 2. The method of claim 1, wherein the default occupancy percentage that is assigned to at least one of the RNs is determined by using a channel occupancy decision rule that would be employed if each of the RNs were to employ the different RAT.
 3. The method of claim 1, wherein the one or more co-existence principles ensure that the RNs should not interfere with the nodes employing the different RAT any more than would another node employing the different RAT.
 4. The method of claim 1, wherein the TDMA technique employs a CSAT algorithm.
 5. The method of claim 1, wherein the RAN operates in accordance with LTE-U and the shared channel is an unlicensed channel.
 6. The method of claim 1, further comprising detecting an external network topology of the one or more nodes that employ the different RAT.
 7. The method of claim 6, wherein the external network topology is detected using REM scans.
 8. The method of claim 6, wherein the channel is also shared by a second RAN that operates in accordance with the RAT employed by the first RAN, the second RAN being controlled by a different network operator than the first RAN, and further comprising using the central controller to determine an external network topology of external RNs in the second RAN and which are able to interfere with one or more of the RNs in the first RAN.
 9. The method of claim 8, wherein determining the default occupancy percentage and any increases in the default occupancy percentages are based at least in part on the external network topology that is determined for the one or more nodes that employ the different RAT and for the external RNs in the second RAN.
 10. The method of claim 8, wherein determining the external network topology of the external RNs in the second RAN includes using the central controller to sequentially cause each of the RNs in the first RAN to be OFF during a normally ON portion of the time periods so that the RNs are able to detect transmissions from the external RNs in the second RAN while the RNs in the first RAN are OFF.
 11. The method of claim 9, further comprising determining if a link exists between one of the nodes that employ the different RAT and the external RNs in the second RAN such that they are able to interfere with one another.
 12. The method of claim 11, wherein determining if the link exists includes using the central controller to select at least one pair of the RNs that are each able to detect transmissions from at least one common one of the external RNs and at least one common one of the nodes that employs the different RAT and using one of the RNs in the pair to transmit while varying its transmission start time and using the other of the RNs in the pair to detect signals from the node that employs the different RAT.
 13. The method of claim 1, further comprising adjusting, without changing the occupancy percentages that are assigned, a transmission start time for the first RN during the time periods so that an overlapping period of a transmission time for the first RN with any transmissions from one or more of the nodes that employ a different RAT and that are able to interfere with the first RN is reduced.
 14. The method of claim 1, wherein the nodes employing the different RAT are Wi-Fi Access Points.
 15. A centralized controller operable in a cell in a small cell radio access network (RAN), comprising: one or more processors; a network interface operatively coupled to the one or more processors and arranged for bidirectional communications with a plurality of radio nodes (RNs) in the RAN; and memory operatively coupled to the one more processors, the memory storing computer-readable instructions which, when executed by the one or more processors, implement a method for assigning a percentage of time to each of the RNs in the plurality of RNs, the percentage of time being a percentage of time during which each of the RNs are able to occupy a channel shared by nodes employing a different radio access technology (RAT) from a RAT employed by the first RAN, comprising: (i) for each repetitive period defined by a time-division multiplexing access (TDMA) technique during which the RNs share a channel with one or more nodes that employ the different RAT, assigning, with the central controller, a default occupancy percentage of the time periods to each of the RNs during which the RNs are able to occupy the channel; (ii) determining, with the central controller, if the default occupancy percentage for a first of the RNs is able to be increased without violating one or more co-existence principles pre-established for the RAT employed by the RNs in the first RAN and the different RAT employed by the one or more nodes; (iii) assigning, with the central controller, an increased occupancy percentage to the first RN if it is determined that the default occupancy percentage for the first RN is able to be increased without violating the one or more co-existence principles; and (iv) sequentially repeating (ii)-(iii) for each remaining one of the RNs in the first RAN. 